Concurrent photochemical whitening and darkening of ambient brown carbon

Abstract

Abstract. The light-absorbing organic aerosol (OA), known as brown carbon (BrC), has important radiative impacts, however its sources and evolution after emission remain to be elucidated. In this study, the light absorption at multiple wavelengths, mass spectra of OA and microphysical properties of black carbon (BC) were characterized at a typical sub-urban environment in Beijing. The absorption of BC is constrained by its size distribution and mixing state, being subtracted from total absorption to obtain the absorption of BrC, then by applying the least-correlation of BC absorption with secondary BrC, the absorption contributed by BC, primary BrC and secondary BrC was apportioned. The multi-linear regression analysis on the factorized OA mass spectra indicated the OA from traffic and biomass burning emission contributed to primary BrC. Importantly, the moderately oxygenated OA (O/C=0.62) was revealed to highly correlate with secondary BrC. These OA had higher nitrogen content, in line with the nitrogen-containing functional groups detected by the Fourier transform infrared spectrometer. The photooxidation was found to result in reduced contribution of primary BrC about 20 % but enhanced contribution of secondary BrC by 30 %, implying the concurrent whitening and darkening of BrC. This provides field evidence that the photochemically produced secondary nitrogen-containing OA can considerably compensate some bleaching effect on the primary BrC, hereby causing radiative impacts.

Field evidence of brown carbon absorption enhancement linked to organic nitrogen formation in Indo-Gangetic Plain

“…This hypothesis probably explains the higher MAEWS-BrC-365 values obtained during late night and morning time except afternoon, which recorded the greatest solar flux (Figure 1b). During afternoon time, a recent study 25 suggested that photochemical oxidation can cause concurrent oxygenation (bleaching) of existing BrC chromophores and formation of nitrogen containing BrC chromophores. Our observations during afternoon are supportive to this hypothesis that WS-BrC bleaching dominates any absorption enhancement whatsoever due to formation of nitrogen containing chromophores during afternoon resulting overall decrease in MAEWS-BrC-365.…”

“…The extent of these processes vary temporally and spatially. 10,25,26 Several previous studies have explored the diurnal [25][26][27][28] and spatial 5,29 variation in sources and absorptivity values of BrC globally. However, field evidence on the role of various atmospheric processes, temperature and RH in governing diurnal variation of chemical composition and absorptivity values of BrC are sparse, especially over Indo-Gangetic Plain (IGP).…”